U.S. Pat. No. 3,785,342 discloses a self cleaning aquarium having a water tank extending over the floor of the tank with a perforated plate extending over the drainage plate. An aggregate material is deposited onto the perforated plate. Water is piped from the drainage plate to a filter located externally of the tank and filtered water from the filter is returned into the tank.
U.S. Pat. No. 5,690,054 discloses an aquarium system consisting of a tank with flanges which support an under gravel filter with side inlet holes and a cleanout. The floor of the tank is shaped to provide one or more depressed areas into which waste is directed by gravity and water flow and from which a drain system allows the waste and stale water to be removed without the need for vacuuming. A filter tower extends upwardly from the filter and returns filtered water into the tank.
Australian Patent 28583/95 discloses an integrated under gravel filter for an aquarium which in many respects is similar to the arrangement illustrated and described in U.S. Pat. No. 3,785,342.
U.S. Pat. No. 4,995,980 discloses an arrangement for biological purification of water in which water within a tank is separated into a space above and a space below a partition. The partition consists of sand or gravel sandwiched between two screens. A diffuser aerates and agitates the water in the space above the partition. The water below the partition has a low oxygen content and has above it a layer supersaturated in oxygen occupied by microalgae and aerobic microorganisms.
With all of these known systems it is periodically necessary to completely drain the tank and clean the filter and/or aggregate material and then refill the tank with clean water. When the tank is drained for cleaning purposes the fish or life forms in the tank need to be removed. This is tedious and may lead to injury to the fish or other life forms in the tank.